It is well known that a piece of woven fabric consists of two sets of threads running at right angles to each other. A weaving machine comprises a roll of warp threads on a warp beam that feeds the warp threads to the healds of the loom, creating a shed of them by raising and lowering them as necessary in preparation for creating the desired weave pattern. The weft thread is drawn laterally through the shed by a shuttle, gripper, air-jet or other means. After the weft has been threaded through the shed, the woven cloth is beaten up and eventually exits the loom part of the weaving machine onto a take-up roller and is then fed to a cloth roller for storage.
Letting off the warp threads from the warp beam is an important aspect in the weaving process as the tension in the warps has a direct effect on the quality of the final cloth. Similarly, precision in taking up the cloth by means of the take-up roller is also an important aspect in the quality of the cloth. The taking up operation has to be arranged so that the finished cloth is drawn from the loom by a carefully controlled amount at a precise moment. Due to the intermittent motion in weaving, as the weft threads are inserted into the shed, this take-up involves drawing the cloth forward in small increments of position. The magnitudes of the increments are related to the thickness of the weft and the required density of the weave.
In automatic weaving, many of the labor intensive functions are performed without the need for operator input. Part of the process of automation has been to control the let-off of the warp threads and the take-up of the cloth according to predetermined control regimes. Early automatic systems used mechanical means to control let-off and take-up, the latter being mechanically locked to the main drive shaft of the loom. More recently, electric motors have been used for let-off and take-up functions, offering increased flexibility and better control of the weaving process.
While these concepts have been well recognized for some time, implementation of electronically controlled let-off and take-up in an automatic weaving machine has proved to be difficult, requiring costly solutions. The control demands associated with let-off and take-up in a typical weaving machine are severe, and require the apparatus positioning the warp beam and the take-up roller rapidly to accelerate and decelerate their considerable inertias (which vary as the weaving process passes the warp from beam to roller). A typical test specification for the drive system might require the electric motor to accelerate between 0 and 2,500 rpm in 0.1 second whilst continuously and accurately following a position reference.
Tensioning the warp beam and positioning the take-up roller has been automated in the past using high performance dc servo motors which have been able to respond both rapidly and accurately to the position and speed control demands. However, dc servo units of the required standard are relatively expensive items. It has not been considered possible in the past to use more simple electric motor systems in these applications (whether ac or dc) because they have been unable to offer the necessary levels of positional accuracy and responsiveness. Furthermore, dc servo units, while being the best available option up to now in spite of the many different forms of electric motor known to exist, suffer from various disadvantages that are exacerbated by the environment in a weaving factory which is likely to be contaminated by airborne dust and fibrous matter.
A conventional dc motor has a wound rotor, excited via a commutator and a set of brushes. This method of commutation is prone to faults in the presence of airborne contamination which means the motors require regular cleaning and/or replacement if they are not to have a significantly increased risk of failure in operation. The inevitable arcing which occurs during commutation is also a fire risk in the contaminated environment of a weaving factory.
More recently, permanent-magnet rotor brushless dc servo motors, utilizing electronic commutation, have been used. These eliminate the problems associated with commutators and brushgear, but are costly. Furthermore, the presence of flammable airborne matter dictates that the servo motor must have a smooth, unfinned exterior surface, so that dust cannot easily accumulate. For the same reason, guard meshes cannot be used to prevent operators from touching the motor housing. Thus, the exterior motor case temperature must, for the safety of the operatives, remain low at all times. The necessity for a low case temperature, together with the enforced absence of cooling fins (or other practicable means of increasing the cooling surface area) means that, in the prior art, a relatively large (and hence costly) servo motor is required, compared with that which would be used in less specialized applications.
The intermittent motion requirements of the weaving machine cause still further cooling problems in any event, due to the high peak torques required to move the relatively large load inertia quickly and accurately. Furthermore, to optimize the dynamic response it is also the practice to supply some types of brushless servo motor continuously with a magnetizing current. This results in a degree of heating of the motor, even when it is not supplying torque to the load. In weaving, this is especially a problem when the motor is required to perform a locking or "holding" function in which it must at all times maintain a set, stationary position of the take-up roller. Conventional motors require the aforementioned magnetizing current to ensure that the servo motor does not permit excessive movement when an external disturbance torque is applied. This continuous magnetizing current leads to further heating and again contributes to the requirement to oversize the motor in order to provide the required performance with acceptable temperature rise of the casing.
Thus, it will be appreciated that the modern automatic weaving machine uses expensive brushless dc servo units for let-off and take-up in the absence of a better alternative rather than because they are inherently suited to the tasks.